Method and device for separating gaseous pollutants from hot process gases by absorption and a mixer for moistening particulate dust

ABSTRACT

In a method of separating gaseous pollutants from hot process gases, the process gases are passed through a contact reactor ( 22 ), in which an absorbent material in a moistened state is introduced to convert the gaseous pollutants into separable dust. The dust is separated in a dust separator ( 10 ). The separated dust is cooled in a first step by being brought into direct contact with a cooling fluid. In a second step, the cooled dust is mixed with a gas containing water vapour, said gas having a saturation temperature that is higher than the temperature of the cooled dust. The dust moistened by condensation of the water vapour is introduced into the contact reactor ( 22 ) to be mixed with the process gases. A mixer ( 24 ) for moistening of absorbent material has a first end ( 26 ) and a second end ( 28 ) and is divided into two zones. A first zone is a cooling zone ( 68 ) located at the first end ( 26 ). A second zone is a moistening zone ( 80 ) located at the second end ( 28 ).

FIELD OF THE INVENTION

The present invention relates to a method of separating gaseouspollutants, such as sulphur dioxide, from hot process gases, such asflue gases, in which method the process gases are passed through acontact reactor, in which a particulate absorbent material reactive withthe gaseous pollutants is introduced in a moistened state into theprocess gases in order to convert the gaseous pollutants into separabledust, after which the process gases are passed through a dust separator,in which dust is separated from the process gases and from which thecleaned process gases are discharged.

The present invention also relates to a mixer for moistening particulatedust which can react with gaseous pollutants in a process gas, such as aflue gas, to form separable dust.

The present invention also relates to a device for separating gaseouspollutants, such as sulphur dioxide, from hot process gases, such asflue gases, said device having a contact reactor, through which theprocess gases are intended to be passed and which has means forintroducing a particulate absorbent material in a moistened state, whichis reactive with the gaseous pollutants, into the process gases for thepurpose of converting the gaseous pollutants into separable dust, and adust separator which is adapted to separate the dust from the processgases and discharge the cleaned process gases.

BACKGROUND ART

When separating gaseous pollutants from process gases, such as fluegases from a coal- or oil-fired fired power plant, a method isfrequently used, in which a lime-containing absorbent material isintroduced into the process gas to react with the gaseous pollutants.When the absorbent material reacts with the gaseous pollutants, thegaseous pollutants are converted chemically or physically into dust,which is then separated in a filter.

WO 96/16722 discloses a method, in which lime-containing dust is mixedwith water in a mixer and then introduced into a contact reactor toreact with gaseous pollutants in a flue gas. Then the dust is separatedin a filter and recirculated to the mixer to be mixed once more withwater and subsequently again be introduced into the contact reactor.Thus, the dust will be circulated through the mixer several times. Ineach circulation, a small amount of dust is removed from the filter, anda small amount of fresh lime-containing material, for instance burntlime, CaO, or dust containing a certain amount of burnt lime, issupplied to the mixer.

It has now been found that the above method results in a low degree ofutilisation of the burnt lime supplied, i.e. that also the dust removedfrom the filter contains some burnt lime, in certain types of absorbentmaterial. This increases the consumption of burnt lime and implies thatthe dust removed from the filter will contain a great deal of burnt limethat has not reacted. The low degree of utilisation of the burnt limeincreases the costs of operating the plant and makes the handling ofremoved dust difficult.

SUMMARY OF THE INVENTION

The object of the present invention therefore is to provide an efficientmethod of separating gaseous substances from process gases, such as fluegases, in which method the above drawbacks of prior-art technique areeliminated or significantly reduced.

This object is achieved by a method which is of the type stated by wayof introduction and characterised in that

a circulating part of the dust separated in the dust separator is cooledin a first step by being brought into direct contact with a coolingfluid,

the cooled dust is mixed in a second step with a gas containing watervapour, said gas having a saturation temperature that is higher than thetemperature of the cooled dust, and

the dust moistened by condensation of the water vapour is introduced asabsorbent material into the contact reactor to be mixed with the processgases.

A further object of the present invention is to provide a mixer fortreating absorbent material intended for separation of gaseoussubstances from process gases, such as flue gases, said mixereliminating or significantly reducing the above drawbacks of prior-arttechnique.

This object is achieved by a mixer which is of the type stated by way ofintroduction and characterised in that the mixer has a first end and asecond end and is divided into two zones, of which a first zone is acooling zone which is located at the first end and which is providedwith a means for supplying a cooling fluid, and of which a second zoneis a moistening zone which is located at the second end and which isprovided with a means for supplying a gas containing water vapour, themixer being adapted first to pass dust from an inlet for dust, locatedat the first end, through the cooling zone and, in the cooling zone,supply a cooling fluid having a lower temperature than the dust, and mixthe dust with this fluid, then pass the dust through the moistening zoneand, in the moistening zone, supply a gas containing water vapour andhaving a saturation temperature which is higher than the temperature ofthe cooled dust, and mix this gas with the cooled dust, and thenintroduce the moistened dust as absorbent material into the process gasthrough an outlet located at the second end.

It is also an object of the present invention to provide a device forseparating gaseous pollutants from a process gas, such as a flue gas,which device eliminates or significantly reduces the above drawbacks ofprior-art technique.

This object is achieved by a device which is of the type stated by wayof introduction and characterised in that it has a cooling zone forcooling at least a circulating part of the dust separated in the dustseparator, means for supplying a cooling fluid to the cooling zone forcooling the dust by direct contact between the fluid and the dust, meansfor feeding the cooled dust to a moistening zone, means for supplying agas containing water vapour and having a saturation temperature which ishigher than the temperature of the cooled dust, to the cooled dust inorder to moisten this by condensation of water vapour, and means forfeeding the moistened dust to the contact reactor.

Further advantages and features of the invention will be evident fromthe following description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail by way of a number ofembodiments and with reference to the accompanying drawings.

FIG. 1 is a schematic side view of a power plant which is provided withequipment for cleaning flue gases.

FIG. 2 is a schematic cross-sectional view of a mixer according to FIG.1 in detail.

FIG. 3 is a schematic cross-sectional view of a mixer according to asecond embodiment of the invention.

FIG. 4 is a schematic cross-sectional view of a third embodiment of theinvention.

FIG. 5 is a schematic cross-sectional view of a fourth embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the invention, dust is treated in order to be introduced into aprocess gas, such as a flue gas. The dust is of a type reacting withgaseous pollutants contained in the process gas and forms, with these,separable dust. The dust is separated and can then be circulated whollyor partly to be treated again and once more be introduced into the fluegas to react with further gaseous pollutants.

According to the invention, the dust is treated in two steps. In a firststep, the dust is cooled by direct contact with a cooling medium to asuitable temperature. In a second step, a gas is supplied, whichcontains water vapour and has a saturation temperature which is higherthan the temperature of the cooled dust. In the second step, watervapour will condense on the cooled particles in the dust. Thus, theparticles in the dust that has passed the second step will be coatedwith a film of condensed water. This film of condensed water has beenfound to give surprising improvements of the treated dust's capabilityof reacting with and binding gaseous pollutants. A possible explanationof the dust's improved capability of reacting with gaseous substancescan be that the condensed film is thin and even and is well suited todissolve gaseous pollutants which can then participate in reactions withthe dust. Another possible explanation of the improved reactivity of thedust is that the film of condensed water causes chemical reactions inthe individual particle. The chemical reactions develop heat and breakthe particle so that its interior will be available for chemicalreactions. One example is dust containing burnt lime, CaO, whichaccording to this explanation can be activated, i.e. be made moreavailable for chemical reactions, such as the slaking reaction that willbe described below.

The direct cooling of the dust in said first step has severaladvantages. The cooling will be efficient by the contact surface betweenthe cooling medium and the particles contained in the dust being great.The cooling fluid supplied to the dust can often be given additionaltasks. Examples of such fluids are compressed air, which simultaneouslywith the cooling can be used to fluidise the dust, and water, whichsimultaneously with the cooling can be used to slake burnt lime, CaO,contained in the dust. By letting one of these fluids in direct contactwith the dust cool the dust in said first step, very efficient coolingis thus achieved by simple means. Indirect cooling with heat exchangersor the like is unsuitable since the risk is great that a heat exchangerwill be clogged in connection with the type of dust that is involved incleaning of process gases.

In the invention, moistened dust is thus supplied to the process gasesand is allowed to react with the process gases. In the reaction, gaseouspollutants are bound to the particles contained in the dust and thusconverted into separable dust. This dust is separated in a dustseparator and collected in a dust hopper. Some of the dust collected inthe dust hopper is removed for landfilling or processing. The major partof the collected dust is supplied to a mixer where it is cooled andmoistened to be introduced once more in the form of absorbent materialinto the process gases. The mixer is continuously supplied with acertain amount of fresh absorbent material to compensate for theabsorbent material consumed in reaction with the gaseous pollutants. Thefresh absorbent material suitably contains burnt lime, CaO. Burnt limecan be supplied in various forms. Examples of fresh absorbent materialsare fresh burnt lime and fly ash from boiler injection. It is quitecommon to inject limestone, CaO₃, directly into a boiler, for instancean oil boiler, for the purpose of separating sulphur dioxide. Thiscleaning method, however, is inefficient and the fly ash separated in adust separator arranged after the boiler contains particles having asurface of gypsum and a core of burnt lime, CaO. It has now surprisinglybeen found possible to reuse, by means of the present invention, fly ashfrom boiler injection as fresh absorbent material. A possibleexplanation of this is that the combination of direct cooling andcondensation of vapour makes it possible to activate the burnt limeinside the particles. Thus, fresh absorbent material can be produced onthe one hand by limestone being supplied to the boiler and forming burntlime which is then collected in the dust separator and passed to thecooling zone and the moistening zone and, on the other hand, bycollected fly ash from another power plant, which uses boiler injectionof limestone, being supplied to the mixer. It has also been foundpossible to use fly ash originating from boilers that are fired withlime-containing fuels, as fresh absorbent material.

Furthermore it has been found possible to use in the invention freshburnt lime of types that could previously not be used. Examples of suchtypes of burnt lime are dead-burnt lime and burnt lime which has beenproduced from limestone containing large amounts of magnesium as apollutant.

The sum reaction when burnt lime, CaO, is used to convert gaseoussulphur dioxide, SO₂, into separable dust can be written as follows:CaO(s)+SO₂(g)+½O₂=>CaSO₄(s)

As is evident from that stated above, one mole of CaO is thus requiredto separate one mole of SO₂ under ideal conditions; this condition iscalled a stoichiometry=1.0. In practice, the number of moles of addedCaO must often be considerably larger than the number of moles of SO₂ toachieve the desired separation of SO₂. A stoichiometry=2.0 means thatthe number of moles of CaO that is supplied is twice as great as thenumber of moles of SO₂ contained in the uncleaned gas. It has been foundpossible to separate, by means of the present invention, a certainamount of SO₂ with a considerably lower stoichiometry, i.e. a smalleraddition of CaO, than has previously been possible in prior-arttechnique.

The fact that the treated absorbent material has increased reactivitywith the gaseous pollutants has also the advantage that the amount ofdust that must be circulated through the mixer can be reduced withoutdecreasing the separation of gaseous pollutants from the process gases.A mixer for use in the invention can therefore be made small andconsumes less energy than in the case of prior-art technique.

It is particularly convenient to use water as a cooling fluid in thecooling zone. Water is cheap and can easily be cooled, if necessary, toa temperature which is lower than the temperature of the separated dust.If the fresh absorbent material contains burnt lime, water will slakethe lime according to the slaking reactionCaO+H₂O=>Ca(OH)₂+heatIn the slaking reaction, heat is thus generated and water is consumed.The slaking of burnt lime in the cooling zone has several advantages.The slaked lime, Ca(OH)₂, is more reactive with regard to the gaseouspollutants than burnt lime, which means that the absorbent material'scapability of binding these pollutants will be improved. A furtheradvantage is that the supplied water in the cooling zone slakes theburnt lime which is easily available on the surface of the particlescontained in the dust. When the dust reaches the moistening zone, thewater that is condensed from the supplied vapour will not be consumed bythe slaking reaction, nor will heat that can evaporate the thin waterfilm develop to any great extent. As a result, the water condensed onthe particles in the moistening zone can form an even and thin filmwhich has good capability of dissolving gaseous pollutants. The even andthin film also seems to be able to penetrate into the particles throughmicro-cracks so as then to break the particles by a slaking reaction inthe interior of the particles, so that their interior of burnt lime willbe available for slaking when passing through the cooling zone duringthe next dust circulation cycle.

Compressed air is a further example of a cooling fluid that can be usedin the cooling zone. Compressed air generated by a compressor oftenkeeps a relatively high temperature, and it is therefore convenient tocool the compressed air to a low temperature. Cooling can take place,for instance, using an air- or water-cooled heat exchanger or directlyby mixing with cold air or cold water. The cooled compressed air can beused, for instance, in the actual mixer as fluidising air or in thevessel collecting the dust separated in the dust separator.

A suitable mixer has a first end and a second end. The inlet for dustfrom the dust separator and the cooling zone are suitably located at thefirst end while the moistening zone and an outlet for moistenedabsorbent material to be introduced into the process gases are suitablylocated at the second end. It has been found appropriate that the dustbe transported in an essentially horizontal direction from the coolingzone to the moistening zone. However, it is also possible to let thetransport of dust through the mixer take place at a certain angle orvertically. It is very important for absorbent material that has beenmoistened in the moistening zone not to be recirculated to the coolingzone to any great extent. It is often suitable to provide the mixer witha mechanical stirrer. This stirrer should mix the dust with the coolingfluid and the gas containing vapour respectively, but not mix dust inthe moistening zone with dust in the cooling zone.

The first step, i.e. the direct cooling of the dust, is suitably carriedout during a period of 2-600 s, still more preferred 2-20 s. It has beenfound that residence times in the cooling zone of less than 2 s yieldinsufficient cooling of the particles in the dust. As a result,condensation of vapour in the subsequent moistening step will beinefficient. With residence times of more than 600 s, also the interiorof the particles will be cooled, which causes the supplied cooling fluidto be used inefficiently. Moreover, longer residence times have thedrawback that a larger mixer is required. The dust cooled in the coolingzone should be transferred to the moistening zone within 10 s. Withlonger periods, the hot interior of the particles will again heat thesurface of the particles, which deteriorates the condensation of vapourin the moistening step.

The second step, i.e. moistening of the dust, is suitably carried outduring a period of 2-30 s on average. It has been found that residencetimes in the moistening zone of less than 2 s yield uneven andinsufficient condensation of water vapour on the particles in the dust.Residence times in the moistening zone of more than 30 s have thedrawback that heat from the interior of the particle is conducted to thesurface of the particle and may cause evaporation of the condensed, thinwater film. A long residence time also requires a large mixer, whichincreases the investment costs and causes more fluidising air to beconsumed. The moistened absorbent material should be introduced into theprocess gases within 5 s for the condensed, thin water film not to beevaporated when heat from the interior of the particle is conducted toits surface.

The gas containing water vapour and having a saturation temperaturewhich is higher than the temperature of the cooled dust is suitablyproduced by compressed air and water vapour being mixed in suitableproportions. Use of water vapour only is also conceivable. Since the gasis often used to fluidise the dust during moistening, relatively largeamounts of gas are required. For this reason, it is in many casesconvenient to add a certain amount of compressed air and dilute thewater vapour with this.

Conveniently the gas containing water vapour has a saturationtemperature with regard to water vapour which is 5-30° C. higher thanthe temperature of the cooled dust. If the saturation temperature of thegas is less than 5° C. higher than the temperature of the dust,condensation will be slow because of a weak driving force. There is alsoa great risk that not the entire particle is coated with a thin film ofcondensed water. When the saturation temperature of the gas is more than30° C. higher than the temperature of the cooled dust, there is a riskthat condensation will be so quick that the dust will locally be wetand/or sticky in some parts of the mixer while the dust is still dry inother parts of the mixer.

According to an embodiment of the invention, a certain amount of wateris supplied directly to the contact reactor where the moistened dust andthe process gases, for instance the flue gases, are mixed. A particularadvantage is then that, for instance, burnt lime, which has beenactivated in the moistening zone by particles being broken, directly inthe contact reactor can be slaked to slaked lime by the added water soas then, almost immediately, to react with the gaseous pollutants. Thus,the moistened dust's capability of reacting with gaseous pollutants isfurther increased. The water supplied directly to the contact reactorcan wholly or partly replace the water supplied to the dust in thecooling zone. For instance, it is possible to use in the cooling zonemerely cooled compressed air, a combination of cooled compressed air andwater, or merely water having a temperature which is lower than thetemperature of the dust collected in the precipitator, in which case thewhole, or parts, of the total amount of water that is required to slakethe burnt lime and provide a suitable temperature and moisture contentin the contact reactor, is supplied directly to the contact reactor.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows schematically a power plant 1. The power plant 1 has acoal-fired boiler 2 in which coal is burnt in air. Air is fed by meansof a fan 4 through a preheater 6 into the boiler 2. Flue gasescontaining dust, such as fly ash, and gaseous pollutants, such assulphur dioxide, are produced in burning. The hot flue gases are passedin a duct 8 through the preheater 6, in which the flue gases exchangeheat with the combustion air coming from the fan 4, to a dust separatorin the form of an electrostatic dust separator 10, also referred to aselectrostatic precipitator. Dust is separated from the flue gas in theprecipitator 10. The precipitator 10 has three dust hoppers 12collecting the dust separated in the precipitator 10.

The cleaned flue gas leaves the precipitator 10 through a duct 14 and isfed by means of a fan 16 through a duct 18 to a chimney 20 to be emittedinto the atmosphere.

The duct 8 has a vertical duct portion which constitutes a contactreactor 22. A mixer 24 communicates with the contact reactor 22 in thelower part thereof.

FIG. 2 illustrates the mixer 24 in more detail. The mixer 24 has a firstend 26 and a second end 28. A circulating part of the dust collected inthe dust hoppers 12 is fed through a duct 30 to an inlet 32 located atthe first end 26 of the mixer 24. A small amount of the dust collectedin the dust hoppers 12 is fed through a duct 34 shown in FIG. 1 forprocessing or landfilling. Fresh absorbent material in the form of burntlime, CaO, is fed through a duct 36 into the mixer 24 through anabsorbent inlet 38 located at the first end 26 of the mixer. At thesecond end 28, the mixer 24 has an outlet 40 formed as an overflowintended for moistened absorbent material and located on one long side.A corresponding outlet for moistened absorbent material is formed on theother long side which is not shown in FIG. 2. Part of the second end 28of the mixer 24 is placed inside the contact reactor 22 in such a mannerthat moistened absorbent material leaving the mixer 24 through theoutlet 40 will be entrained by the flue gas flowing upwards in thecontact reactor 22.

The mixer 24 has a mechanical stirrer 42 which has a shaft 44 extendingfrom the first end 26 to the second end 28 and journalled in bearings inthe respective ends 26, 28. A motor 46 is arranged to rotate the shaft44 and the inclined, elliptic discs 48 that are fixedly connected to theshaft.

A gas-permeable cloth 52 is fixed in the lower portion 50 of the mixer24. A partition 54 extends from the bottom 56 of the mixer 24 to thecloth 52 and thus divides the space between the cloth 52 and the bottom56 into a first chamber 58 and a second chamber 60. An array of nozzles62 is arranged inside the mixer 24 above the first chamber 58. A waterpipe 64 is arranged to supply water having a lower temperature than thedust that is supplied to the mixer 24 through the dust inlet 32, to thenozzles 62. An air line 66 is arranged to supply compressed air to thechamber 58. The space of the mixer 24 which at the first end 26 thereofis formed between the first chamber 58 and the nozzles 62 willconstitute a cooling zone 68 where the dust supplied to the mixer 24 ismixed with the water supplied through the nozzles 62 and is cooled bythe same.

A gas conduit 70 is arranged to supply pressurised gas to the secondchamber 60. The gas conduit 70 is supplied with compressed air throughan air line 72 and water vapour through a vapour line 74. A moisturecontent meter 76 is arranged to measure the moisture content of themixture of pressurised vapour and compressed air. A control unit 78 isarranged to receive a signal from the moisture content meter 76 and, inresponse to this signal, control the supply of water vapour through thevapour line 74 in such a manner that the desired saturation temperatureis obtained in the gas supplied to the second chamber 60 through the gasconduit 70. The saturation temperature of the gas supplied through thegas conduit 70 is controlled in such a manner that the saturationtemperature at the pressure prevailing in the mixer 24 is higher thanthe temperature of the dust that has been cooled in the cooling zone 68.

When operating the mixer 24, dust from the precipitator 10 and freshabsorbent material will continuously be supplied to the mixer throughthe dust inlet 32 and the absorbent inlet 38 respectively. Since thecloth 52 is permeable to gas, the compressed air supplied through theline 66 will fluidise the supplied dust above the first chamber 58. Thesupply of dust through the dust inlet 32 at the first end 26 of themixer 24 and the fluidisation will cause the dust to be transported, inFIG. 2 indicated by an arrow P, from the first end 26 of the mixer 24 inthe direction of its second end 28 during mixing and cooling in thecooling zone 68. The gas supplied through the gas conduit 70 willfluidise the cooled dust above the second chamber 60. Since thesaturation temperature of the gas supplied through the gas conduit 70 ishigher than the temperature of the cooled dust, water vapour willcondense on the dust. Thus at the second end 28 of the mixer 24 amoistening zone 80 located above the second chamber 60 will be provided,where the dust that has been cooled in the cooling zone 68 is moistenedby water vapour supplied through the gas conduit 70 condensing on thesurface of the particles contained in the dust. The moistened dust istransported through the mixer 24 in the direction of arrow P and leavesthe mixer 24 through the outlet 40. Since the outlet 40 is in directcommunication with the contact reactor 22, the moistened dust will bemixed with the flue gases and react with gaseous pollutants contained inthe flue gases.

FIG. 3 shows a second embodiment of the present invention in the form ofa mixer 124. The mixer 124 differs from the mixer 24 as described abovemainly with regard to the design of the cooling zone. Thus the coolingzone 168 of the mixer 124 also comprises a spray reactor 169. The sprayreactor 169 has a nozzle 162 and a water pipe 164 which is arranged tosupply water to the nozzle 162. Dust from the dust hoppers 12 issupplied to the spray reactor 169 through an inlet 131 located at theupper end thereof. The spray reactor 169 is thus arranged to mix water,that has a lower temperature than the dust separated in the precipitator10, with the circulating part of the dust and, consequently, cool thedust. In the lower part of the spray reactor 169 there is an outlet 132,through which the cooled dust is supplied to the mixer 124 at the firstend 126 thereof. The mixer 124 has, like the mixer 24, a horizontalgas-permeable cloth 152 just above its bottom 156. A partition 154divides the space between the cloth 152 and the bottom 156 into a firstchamber 158 located at the first end 126 of the mixer 124, and a secondchamber 160 located at the second end 128 of the mixer 124. A compressedair line 166 is arranged to supply compressed air to the first chamber158 and to fluidise and mix the cooled dust coming from the sprayreactor 169. The space of the mixer 124 which is located above the firstchamber 158 will thus, together with the spray reactor 169, form acooling zone 168. A gas conduit 170 is arranged to supply to the secondchamber 160 a mixture of compressed air supplied from a compressed airline 172 and water vapour supplied through a vapour line 174 in a mannersimilar to that described above with regard to the mixer 24. The spaceof the mixer 124 which is located above the second chamber 160 will thusform a moistening zone 180. As indicated in FIG. 3, a heat exchanger 167can be arranged on the compressed air line 166. The heat exchanger 167,which suitably uses cold water as cooling medium, can be used to coolthe compressed air that is to be supplied to the first chamber 158 to atemperature that is lower than the temperature of the dust coming fromthe spray reactor 169. As a result, the thus cooled compressed air willprovide additional cooling of the dust in the space of the mixer 124located above the chamber 158.

FIG. 4 shows a third embodiment of the invention in the form of a mixer224. The mixer 224 is essentially identical with the mixer 24 shown inFIG. 2 and thus has a first chamber 258 and a second chamber 260. Acompressed air line 266 is arranged to supply compressed air to thefirst chamber 258 for the purpose of fluidising dust. A gas conduit 270is arranged to supply to the second chamber 260 a mixture of compressedair, which is supplied from a compressed air line 272, and water vapour,which is supplied through a vapour line 274, for the purpose offluidising and moistening the dust in a manner similar to that describedabove with regard to the mixer 24 before the moistened dust isintroduced as absorbent material into the contact reactor 22 (not shownin FIG. 4). A water pipe 264 is arranged to supply water to a number ofnozzles 262 arranged above the first chamber 258. The dust hoppers 12 inthe precipitator 10 open into a fluidised collecting vessel 214. Thecollecting vessel 214 has a gas-permeable cloth 216. A compressed airline 218 is arranged to supply compressed air to an air chamber 222formed between the cloth 216 and the bottom 220 of the vessel 214, forthe purpose of fluidising in the vessel 214 the dust that has beencollected in the dust hoppers 12. The compressed air line 218 isprovided with a heat exchanger 225. Water having a temperature that islower than the temperature of the dust separated in the precipitator 10is supplied to the heat exchanger 225 through a pipe 227. The heatexchanger 225 has such a heat transfer surface that compressed airsupplied through the line 218 can be cooled to a temperature that islower than the temperature of the dust separated in the precipitator 10.The cooled compressed air supplied to the vessel 214 will cool the dustduring fluidisation. The dust cooled in the vessel 214 is then suppliedthrough a dust line 230 and an inlet 232 located at the first end 226 ofthe mixer 224, to the mixer 224 so as then to be further cooled by thewater supplied to the mixer 224 through the nozzles 262. A line (notshown) is arranged to remove a small amount of the dust collected in thevessel 214 for landfilling. The embodiment shown in FIG. 4 will thushave a cooling zone 268 which consists on the one hand of the collectingvessel 214 and, on the other hand, of the space of the mixer 224 whichis located above the first chamber 258. The space of the mixer 224located above the second chamber 260 will also in this embodiment form amoistening zone 280, which opens into an outlet, not shown in FIG. 4,for moistened dust.

FIG. 5 shows a fourth embodiment of the invention. In this embodiment, acollecting vessel 314 is used as a cooling zone 368, and a mixer 324 isused in its entirety as a moistening zone 380. The dust hoppers 12 inthe precipitator 10 open into the fluidised collecting vessel 314 whichhas a gas-permeable cloth 316. A compressed air line 318 is arranged tosupply compressed air to an air chamber 322 formed between the cloth 316and the bottom 320 of the vessel 314, for the purpose of fluidising thedust that has been collected in the dust hoppers 12. The compressed airline 318 is provided with a heat exchanger 325. Water having atemperature which is lower than the temperature of the dust separated inthe precipitator 10 is supplied to the heat exchanger 325 through a pipe327. The heat exchanger 325 has such a heat transfer surface thatcompressed air supplied through the line 318 can be cooled to atemperature which is lower than the temperature of the dust separated inthe precipitator 10. The cooled compressed air supplied through the line318 to the vessel 314 will cool the dust during fluidisation. The dustcooled in the vessel 314 is then fed through a dust line 330 and aninlet 332 located at the first end 326 of the mixer 324, into the mixer324.

A gas-permeable cloth 352 is fixed in the lower portion 350 of the mixer324. The space between the cloth 352 and the bottom 356 of the mixer 324forms a chamber 360. A gas conduit 370 is arranged to supply pressurisedgas to the chamber 360. The gas conduit 370 is supplied with compressedair through an air line 372 and water vapour through a vapour line 374.The saturation temperature of the gas is controlled in the way describedabove with reference to FIG. 2 so that the saturation temperature iskept higher than the temperature of the cooled dust. Since thesaturation temperature of the gas supplied through the gas conduit 370is higher than the temperature of the cooled dust, water vapour willcondense on the particles contained in the dust in the moistening zone380. The thus moistened particles leave the mixer 324 through an outlet340 for moistened dust which is located at the second end 328 of themixer and formed as an overflow. The dust is mixed with a flue gasgenerated in a boiler (not shown in FIG. 5). The boiler is provided witha device for boiler injection of limestone, CaCO₃. Thus the flue gasesgenerated in the boiler and supplied through a duct 308, which via theoutlet 340 communicates with the mixer 324, will also contain some burntlime, CaO, which has been produced in the boiler as a result of theboiler injection. In the embodiment shown in FIG. 5, the burnt limeproduced in the boiler will thus constitute an addition of freshabsorbent. When the flue gas in the duct 308 passes by the outlet 340,it will entrain the moistened dust which via the outlet 340 leaves themixer 324. Then the flue gas mixed with moistened dust reaches a contactreactor in the form of a fluidised bed 322. Moistened dust, flue gas andwater, which by means of a water pipe 364 and a nozzle 362 is injectedinto the lower portion 321 of the bed 322, are mixed in the fluidisedbed 322. The nozzle 362 is designed so that the water is atomised andmixed with the flue gases and the moistened dust. Thus, the burnt lime,which by the treatment in the moistening zone 380 has been activated byparticles being broken, can be slaked by the water injected into the bed322 by means of the nozzle 362, and thus immediately react with gaseouspollutants. The flue gas and the particles are then passed through aduct 323 from the fluidised bed 322 to the precipitator 10 where theparticles are separated and again passed to the collecting vessel 314for renewed cooling in the cooling zone 368 and subsequent moistening inthe moistening zone 380. A small part of the dust collected in theprecipitator 10 is continuously removed in order to compensate for freshabsorbent material being supplied continuously. For instance, dust canbe removed from the hoppers 12, in which case the removed amount neednot be cooled in the vessel 314, from the actual vessel 314 or from theline 330 through a discharge line not shown in FIG. 5.

It will be appreciated that many modifications of the embodimentsdescribed above are conceivable within the scope of the invention asdefined by the appended claims.

For instance, it is possible to use merely cooled air in the directcooling of the dust also in other embodiments than the embodimentillustrated in FIG. 5. In such a case, it is possible in the embodimentshown in FIG. 4 to exclude the first chamber and the nozzles and makethe mixer shorter. Thus, the cooling zone is located entirely in thecollecting vessel, only the moistening zone being located in the actualmixer. It is also possible to supply some water to the collecting vesselin order to further increase the direct cooling. In some cases, thetemperature of the supplied compressed air is sufficiently low even atthe beginning, i.e. lower than the temperature of the dust that has beencollected in the precipitator, thereby making cooling by means of a heatexchanger unnecessary. In such cases, the heat exchanger 225 and 325,respectively, can thus be excluded in the embodiments shown in FIG. 4and FIG. 5 respectively.

The above-described electrostatic precipitator can alternatively consistof some other suitable dust precipitator, such as a fabric filter, forinstance a bag filter, or a cyclone or some other filter that issuitable for separation and recirculation of particulate material.

The addition of water to the actual contact reactor, as shown in FIG. 5,can also be made in the embodiments shown in FIGS. 2-4. Thus, forinstance a nozzle can be placed in the contact reactor 22 in theembodiment shown in FIG. 2 to inject and mix water with the moisteneddust and the flue gases. This is advantageous since burnt lime, whichhas been activated in the moistening zone 80, can be slaked directly inthe flue gas by means of the water injected into the contact reactor.This means that the burnt lime which has been activated in themoistening zone can even in the first mixing with the flue gas reactwith gaseous pollutants.

The stirrer of the mixer can be designed in various ways. A preferreddesign is a longitudinal paddle shaft according to FIG. 2, or aplurality of longitudinal paddle shafts. Another preferred designinvolves the parallel paddle shafts as described in U.S. Pat. No.6,213,629. It is also possible to do without mechanical stirrers in themixer and instead let the fluidising compressed air or the fluidisinggas containing water vapour effect the stirring in the mixer. Theimportant thing is that satisfactory mixing of water or gas with thedust should be provided and that the dust in the moistening zone shouldnot be remixed with the dust in the cooling zone to any great extent.

EXAMPLE 1

An experiment was carried out by means of a mixer 24 of the type shownin FIG. 2. A synthetic flue gas having a sulphur dioxide, SO₂, contentof 1000 ppm was used. The flue gas was fed through a contact reactor 22past the outlet 40 of the mixer 24, after which the dust was separatedin a dust separator in the form of a fabric filter. The cleanedsynthetic flue gas had a temperature of about 74° C. The separated dustwas then recirculated to the mixer 24. The temperature of the dust,before being supplied to the mixer 24, was about 72° C., i.e. somewhatlower than the temperature of the cleaned flue gas. On average, the dustwas circulated 35 times through the mixer, the contact reactor and thefilter before being removed for disposal.

As fresh absorbent, a fly ash was used, collected from a coal-firedpower plant, which had boiler injection of limestone, which means thatlimestone, CaCO₃, was supplied directly to the boiler. The fly ash had acontent of available burnt lime, CaO, of 30% by weight (measuredaccording to ASTM C25). The supply of fresh absorbent in the form of flyash was set at a stoichiometry of 1.6, i.e. for each mole of SO₂ in theuncleaned flue gas, 1.6 mole of CaO in the form of fly ash was suppliedto the mixer.

Water was supplied to the cooling zone 68 of the mixer. The water had atemperature of about 10° C. The quantity of water in kg/s was about onetwentieth of the amount of recirculated dust in kg/s. When passingthrough the cooling zone, the average temperature of the dust waslowered by about 2-3° C., but locally on the surface of the particles,the temperature may be assumed to have been reduced to a significantlygreater extent. The average residence time of the dust in the coolingzone was about 10 s.

The cooled dust then left the cooling zone and was almostinstantaneously, within 1 s, subjected to the moistening zone 80. Thegas which was supplied to the moistening zone through the gas conduit 70was a mixture of compressed air and water vapour and had a temperatureof 86° C. and was saturated at this temperature (the moisture content ofthe gas was about 60%). Thus, a great amount of water vapour wascondensed on the particles of the dust in the moistening zone of themixer. The average residence time of the dust in the moistening zone wasabout 15 s. The dust was then supplied as moistened absorbent materialalmost instantaneously, within 1 s, to the contact reactor 22 throughthe outlet 40. Measurements proved that about 84% of the SO₂ that waspresent in the uncleaned flue gas was converted into separable dust, andonly 16% of the total amount of SO₂ was to be found in the cleaned fluegas.

EXAMPLE 2

In Example 2, a mixer 24 of the same type as in Example 1 was used. Alsothe other experimental parameters were the same as in Example 1, exceptthat the cleaned flue had a lower temperature, more specifically atemperature of 65° C. Moreover, the flue gas in Example 2 had atemperature which was only 11° C. higher than the current saturationtemperature of the flue gas, compared with 14° C. higher in Example 1.These two differences, which were due to differences in the generationof the synthetic flue gas, would, under otherwise identical conditions,be expected to improve the separation of SO₂ in Example 2 compared withExample 1.

Only compressed air was supplied to the “moistening zone” of the mixer,no water vapour. The gas which thus contained only compressed air andwhich was supplied to the moistening zone through the gas conduit 70 hada saturation temperature of about 12° C., which corresponds to amoisture content of about 1.5%, and a temperature of 86° C. The dust hada temperature of about 63° C. when being fed to the cooling zone of themixer. In this Example, hardly any condensation of water vapour at allthus occurred in the “moistening zone” of the mixer. Measurements provedthat only about 61% of the SO₂ that was present in the uncleaned fluegas was converted into separable dust, and as much as 39% of the totalamount of SO₂ was to be found in the cleaned flue gas.

EXAMPLE 3

Under essentially the same conditions as in Example 2, an attempt wasmade to increase the stoichiometry, i.e. the amount of added burnt lime,CaO, in relation to the amount of SO₂, for the purpose of increasing theamount of converted and separated SO₂. Therefore the stoichiometry wasincreased from 1.6 to 3.9. However, measurements proved that in spite ofthis greatly increased stoichiometry, only slightly more than 60% of theSO₂ that was present in the uncleaned gas was converted into separabledust.

As is evident from the Examples above, the invention, as exemplified inExample 1, has great advantages as regards the capacity of separatinggaseous pollutants, in particular SO₂, compared with the cases where thegas supplied to the “moistening zone” does not have a saturationtemperature which is higher than the temperature of the dust, which isexemplified in Examples 2 and 3.

1. A method of separating gaseous pollutants, such as sulphur dioxide,from hot process gases, such as flue gases, in which method the processgases are passed through a contact reactor (22; 322), in which aparticulate absorbent material reactive with the gaseous pollutants isintroduced in a moistened state into the process gases in order toconvert the gaseous pollutants into separable dust, after which theprocess gases are passed through a dust separator (10), in which dust isseparated from the process gases and from which the cleaned processgases are discharged, characterised in that a circulating part of thedust separated in the dust separator (10) is cooled in a first step bybeing brought into direct contact with a cooling fluid, the cooled dustis mixed in a second step with a gas containing water vapour, said gashaving a saturation temperature that is higher than the temperature ofthe cooled dust, and the dust moistened by condensation of the watervapour is introduced as absorbent material into the contact reactor (22;322) to be mixed with the process gases.
 2. A method as claimed in claim1, in which the cooled dust contains burnt lime, CaO, which during atleast one of said first and second step at least partly is subject toslaking to slaked lime, Ca(OH)₂.
 3. A method as claimed in claim 1 or 2,in which fresh absorbent material is continuously supplied to theprocess gases, part of the dust separated in the dust separator (10)being removed without being brought into direct contact with the coolingfluid.
 4. A method as claimed in claim 1, in which the dust is cooled insaid first step by being mixed with water, which has a lower temperaturethan the dust separated in the dust separator (10).
 5. A method asclaimed in claim 4, in which said circulating part of the dust separatedin the dust separator is introduced into a mixer (24; 124; 224), whichhas an inlet (32; 132; 232) at a first end (26; 126; 226) and an outlet(40) at a second end (28; 128), said circulating part of the dustseparated in the dust separator (10) being passed horizontally throughthe mixer (24; 124; 224) from the first end (26; 126 226) to the secondend (28; 128) and being mixed with water and cooled in a cooling zone(68; 168; 268) adjacent to the first end (26; 126; 226) of the mixer(24; 124; 224), so as then to be mixed, in a moistening zone (80; 180;280) adjacent to the second end (28; 128) of the mixer (24; 124; 224),with the gas containing water vapour.
 6. A method as claimed in claim 1,in which the dust is cooled in said first step by mixing with air whichhas a lower temperature than the dust separated in the dust separator(10).
 7. A method as claimed in claim 6, in which the air is at leastpartly supplied to a collecting vessel (214) connected to the dustseparator (10) and intended for collecting separated dust.
 8. A methodas claimed in claim 1, in which water is supplied directly to thecontact reactor (322) and is mixed with the moistened dust and theprocess gases.
 9. A method as claimed in claim 1, in which the firststep is carried out during a period of 2-600 s on average, and that thecooled dust is then subjected to the second step within 10 s.
 10. Amethod as claimed in claim 1, in which the second step is carried outduring a period of 2-30 s on average, and that the moistened dust isthen introduced into the process gases within 5 s.
 11. A method asclaimed in claim 1, in which said gas contains air and water vapourwhich are mixed to the desired saturation temperature.
 12. A method asclaimed in claim 1, in which the gas, which contains water vapour, has asaturation temperature which is 5-30° C. higher than the temperature ofthe cooled dust.
 13. A mixer for moistening a particulate dust, whichcan react with gaseous pollutants in a process gas, such as a flue gas,in order to form a separable dust, characterised in that the mixer (24;124; 224) has a first end (26; 126; 226) and a second end (28; 128) andis divided into two zones, of which a first zone is a cooling zone (68;168; 268) which is located at the first end (26; 126; 226) and which isprovided with a means (62, 64; 162, 164, 166, 167; 262, 264) forsupplying a cooling fluid, and of which a second zone is a moisteningzone (80; 180; 280) which is located at the second end (28; 128) andwhich is provided with a means (70; 170; 270) for supplying a gascontaining water vapour, the mixer (24; 124; 224) being adapted first topass dust from an inlet (32; 132; 232) for dust, located at the firstend (26; 126; 226), through the cooling zone (68; 168; 268) and, in thecooling zone, supply a cooling fluid having a lower temperature than thedust, and mix the dust with this fluid, then pass the dust through themoistening zone (80; 180; 280) and, in the moistening zone, supply a gascontaining water vapour and having a saturation temperature which ishigher than the temperature of the cooled dust, and mix this gas withthe cooled dust, and then introduce the moistened dust as absorbentmaterial into the process gas through an outlet (40) located at thesecond end (28; 128).
 14. A mixer as claimed in claim 13, in which themeans (62, 64; 162, 164; 262, 264) for supplying a cooling fluid isarranged to supply water to the dust.
 15. A mixer as claimed in claim 13or 14, in which the mixer has a means (166) for supplying cooledcompressed air to the cooling zone (168).
 16. A device for separatinggaseous pollutants, such as sulphur dioxide, from hot process gases,such as flue gases, said device having a contact reactor (22; 322),through which the process gases are intended to be passed and which hasmeans (24, 40; 124; 224; 324; 340) for introducing a particulateabsorbent material in a moistened state, which is reactive with thegaseous pollutants, into the process gases for the purpose of convertingthe gaseous pollutants into separable dust, and a dust separator (10)which is adapted to separate the dust from the process gases anddischarge the cleaned process gases, characterised in that the devicehas a cooling zone (68; 168; 268; 368) for cooling at least acirculating part of the dust separated in the dust separator (10), means(62, 64; 162, 164, 166; 218, 262, 264; 318) for supplying a coolingfluid to the cooling zone (68; 168; 268; 368) for cooling the dust bydirect contact between the fluid and the dust, means (52; 152; 330) forfeeding the cooling dust to a moistening zone (80; 180; 280; 380), means(70; 170; 270; 370) for supplying a gas containing water vapour andhaving a saturation temperature which is higher than the temperature ofthe cooled dust, to the cooled dust in order to moisten this bycondensation of water vapour, and means (40; 340) for feeding themoistened dust to the contact reactor (22; 322).
 17. A device as claimedin claim 16, in which a means (362, 364) is arranged to inject waterinto the contact reactor (322) and mix this with the moistened dust andthe process gases.